Calls to AudioOutputI2SNoDAC() kill other pins

[KDMcMullan]

When I call

out = new AudioOutputI2SNoDAC();

it appears to do something to pin D8 (GPIO15). It doesn't seem to function well as an input after this call.

[KDMcMullan]

This is believed to be a feature of the
WRITE_PERI_REG(PERIPHS_IO_MUX_MTDO_U, orig_bck);
call in
AudioOutputI2SNoDAC.cpp
I don't fully understand the low level registers, but it is believed this tells GPIO15 to be the SPI chip select. The workaround is to call
pinMode(15, INPUT); // DTMF decoder pins
after the call to the above.